Method of cathodically protecting well casing



A ril 10, 1962 G. A. MARSH ETAL 3,

METHOD OF CATHODICALLY PROTECTING WELL CASING Filed May 29, 1959 PERFORATE WELL CASING PACK WELL CASING BELOW PERFORATIONS PLACE HEATER ADJACENT TO PACKING PLACE PELLETS OF LOW MELTING ALLOY ON PACKING MELT ALLOY PELLETS PPLY PRESSURE TO FORCE MOLTEN METAL THROUGH PERFORATIONS FIG.I

INVENTORS GLENN A. MARSH BY EDWARD SCHASCHL ATTORNEY United rates FatentGiifiQe 3,029,195 Patented Apr. 10, 1962 3,029,195 METHGD F ATH9DI CALLY PRQTEQTING WELL CASENG Glenn A. Marsh and Edward Schaschl, Crystal Lake, .liL,

assignors to The Pure (til Company, Chicago, Ill, a

corporation of Ohio Filed May 29, 1959, Ser. No. 816,942 10 Claims. (U. 294-148) This invention relates to new and useful processes for mitigating corrosion, and more particularly to an improved means for providing cathodic protection of well casings from corrosion at relatively inaccessible points.

it is well recognized that when different metals of the clectromotive series are placed in contact with each other within an electrolytic solution, such as brine or underground water, etc., an electric current flows between the two metals from the metal which serves as the anode to the metal which serves as the cathode. This electrolytic current is associated with the chemical oxidation of the anodic metal. Metal conduits such as oil well tubing and casings, have variations in composition from point to point such that some portions of the metal may be cathodic and other portions anodic at difierent points along the metallic surface. galvanic action at the internal or external surface of such well casings is so extensive that it has become a major problem in the efiicient operation of wells.

Cathodic protection has been developed as a means for mitigating or preventing corrosion in underground structures and the like. In one form, this system of protection may consist of the introduction of a more reactive, sacrificial metallic anode in close proximity to the conduit in the region to be protected. The metallic conduit along its entire surface is transformed from a series of cathodic and anodic portions, differing slightly in the composition of their metallic constituents from point to point, into a single cathode. By more reactive in this context is meant a metal of the electromotive series which, when placed in contact with a less reactive metal of the same series, becomes an anode while the less reactive metal becomes a cathode. The anode, for the purpose of this invention, is defined as the electrode in a galvanic cell at which oxidation occurs. The cathode, similarly, is delined as the electrode in a galvanic cell at which reduction occurs. The introduced sacrificial anode undergoes corrosion, but the anode may be replaced whenever necessary. The conduit or well casing, formerly subject to corrosion at numerous inaccessible points, is protected and preserved by use of the sacrificial anode.

In providing cathodic protection'to well casings, a serious problem has developed as a result of the relative inaccessibility of casings which are located some considerable distance underground. Attempts have been made to provide sacrificial anodes in a form which could be secured in place on the outside of a well casing at a considerable depth below ground. However, such attempts have not been entirely successful and a considerable demand exists for a satisfactory method of providing cathodic protection at relatively inaccessible points in a well casing.

It is therefore one object of this invention to provide an improved means of preventing corrosion of ferrousmetal well-casing at relatively inaccessible points.

Another object of this invention is to provide a means for cathodically protecting an iron or steel well casing at a point which is relatively inaccessible.

A feature of this invention is the provision of an improved means for cathodic protection of well casings by perforating the well casing at a desired point underground and forcing through the perforations relatively large amounts of a molten metallic composition which is an- As a result, corrosion due to such odic to the casing metal, and which, upon solidification, joins electrically with the structure to be: protected and forms a sacrificial anode.

Other objects and features of this invention will become apparent from time to time throughout the specification and claims as hereinafter related.

In the accompanying drawing, FIGURE 1 is a flow diagram indicating the steps of our improved process, and FIG. 2 is a diagrammatic view showing a section of the earth with a well casing in place and our improved means for cathodic protection. 7

According to our invention, an oil-well casing is perforated at the depth where protection is desired, preferably opposite some zone known to be associated with external corrosion of the casing. Then a suitable packer is installed just below the lowest perforations, and an electric heater or other suitable heating means is lowcred on a cable to a level adjacent to the perforations. Following this, the requisite amount of a low-melting metal alloy, in pellet or other suitable form, is dropped into the casing to rest on the packer and surround the electric heater. The heater is energized and the metal is melted, after which gas pressure is applied at the Well head to force the molten metal through the perforation into the formation. The metal used is a low-melting alloy which is anodic to the iron or steel well-casing. In practice, we prefer to preheat the formation surrounding the perforated section of the casing to a point just above the melting point of the anodic metal before dropping the metal into the casing, or before applying gas pressure to the molten metal. This preheating may be accomplished by the electric heater or by combustion in the well here at this point. After the anodic metal has been injected and allowed to solidify, the packer is removed, and excess anodic material which has solidified within the casing is removed by reaming or other conventional method. If desired, excess amounts of the anodic metal which do not interfere with the operation of the well may be left in place to provide cathodic protection against corrosion of the internal surfaces of the well casing.

Referring to the drawing by numerals of reference, there is shown in FIG. 2 a diagrammatic, vertical, sectional view of the earth and a well casing in position. In this drawing, there is shown a plurality of earth formations or strata 1, 2, and 3 through which there has been positioned a well casing 4. At a point within well casing 4, some considerable distance below ground, a suitable packer 5 is positioned. The well is perforated to provide holes 6 in casing 4 just above packer S. The perforation of well casing 4 is accomplished with any conventional perforating device such as a well-perforating gun or the.

like. In preparation for treatment of the well casing, a combustible gas or volatile hydrocarbon may be introduced into the well and burned, adjacent to perforations 6, to heat the well casing and the surrounding formation to a temperature slightly above the melting point of the anodic metal which is to be applied in the well. A suitable heating device, preferably an electric heater 7, is lowered into the well to the level of packer '5. Then a low-melting metal alloy, in pellet or other suitable form, is dropped into the casing to rest on the packer. In general, approximately to 500 pounds of the anodic metal are used in this process. The electric heater 7 is energized and metal pellets 8 are melted. Next, gas pressure, e.g., compressed air, nitrogen. or other inert gas, is applied to the well to force the molten anodic metal out through perforations d to solidify within the surrounding formation as indicated by dotted lines 9. After this metal has been injected and allowed. to solidify, the excess metal within the casing is removed by reaming or other conventional methods and the packer 5 is removed from the well. If desired, part or all of the anodic metal which remains within the well casingmay be left in place, if it does not interfere with operation of the well, to provide cathodic protection for the inner surfa'ceso'f well casings; W v h in carrying out this invention as described in the above example, it is preferred to use low-melting alloys which have melting points below about 600 F. Any metal or alloy which melts below about 606 F., and which is anodic with respect to the metal of the well casing, i.e., iron or steel, is satisfactory for the purpose of this invention providedonly that such metal not be violently reactive with the constituents of the strata into which it is to be injected through the perforations in the well casing. About the only low-melting anodic alloys which are excluded from this invention are ones which consist entirely of, or contain a hi h proportion of, the very active alkali metals. However, alloys of the alkali metals with other metals, in which the alkali metal content is suliiciently small as to make the alloy not violently reactive with water or other constituents of the strata into which the metal is to be injected, are satisfactory. One alloy which may be used in the above-described process is the bismuthgallium alloy which melts at about 440 F. Another alloy which is satisfactory for the purpose of this invention is the gallium-indium alloy, containing about 80-90% indium, which melts in the range between about 190 and 250 F. Sodium-tin, zinc-tin, and lithium-magnesium alloys which melt elow 600 F. are preferred, from purely economic considerations. Other alloys which melt in the desired temperature range and which are satisfactory for the purpose of this invention inciude:

Lithium-mercury Bismuth-lithium Bismuth-potassium Bismuth-sodium Lithium-tin Sodium-tin Zinc-tin It is to be noted that some of the above-listed alloys are not operative for protecting pure iron, since some of these alloys are not anodic with respect to pure iron. However, these alloys are included in the above list since they melt in the desired range and are ancdic with respect to certain alloy steels and thus can be used for protecting such alloys against electrolytic corrosion. Some of the above listed alloys which contain small amounts of the alkali metals are highly desirable in this process since the alkali metals react with moisture in the environment surrounding the well casing and create an alkaline environment around the casing which is effective in stifling the activity of sulfate-reducing bacteria and in neutralizing corrosive acidic constituents such as hydrogen sulfide. While the alloys listed above are all binary alloys, it is to be understood that any alloy can be used in this invention regardless of the number of constituents therein so long as the alloy is anodic with respect to the structure to be protected. Likewise, this inventiondoes not exclude the use of pure metals, e.g., gallium, which have a melting point less than about 600 F. and which are anodic with respect to the structure to be protected. In general, however, alloys are preferred to the pure metals because of their lower melting points. Additional information on low-melting alloys which are considered within the scope of this invention and which may be used in the manner above described may be found in the Liquid Metals Handbook, Richard N. Lyon, 2nd. ed., 1952, published jointly by the Atomic Energy Commission and the Bureau of Ships, Department of the Navy.

While we have described our invention fully and completely with special emphasis upon several preferred embodiments threof, we wish it to be understood that the invention is not limited to the recited embodiment and that within the scope of the appended claims, this invention may be practiced otherwisethan as specifically described.

What is claimed is:

1. A method of providing cathodic protection to the exterior surface of metallic well casings which comprises forming perforations in the casing below ground, placing a low-melting metal anodic to the casing adjacent to sai perforations, melting said metal, and applying gas pres sure to the well to force the molten anodic metal through said perforations to form in-situ, sacrificial metal anodes in electrical contact with said casing.

2. A method in accordance with claim 1 in which the anodic metal used is an alloy melting at a temperature less than about 600 F.

3. A method in accordance with claim 2 in which the low-melting alloy is selected from the group consisting of bismuth-gallium, gallium-indium, bismuth-cad mium, bismuth-lithium, bismuth-potassium, cadmiumlead, cadmium-mercury, cadmium-sodium, cadmium-tin, cadmium-zinc, lead-lithium, lithium-magnesium, lithiurnmercury, bismuth-sodium, lithium-tin, sodium-tin, and Zinc-tin alloys.

4. A method in accordance with claim 2 in which excess amounts of the low-melting alloy are removed from the casing bore after resolidification.

5. A method in accordance with claim 2 in which the alloy is melted with an electric heater.

6. A method of providing cathodic protection to the exterior surface of well casings formed of ferrous metal which comprises forming perforations in the casing below ground, placing a packing in said casing just below said perforations, placing heating means at said packing, placing on said packing a low-melting metal anodic to the casing, melting said anodic metal with said heating means, and applying gas pressure to said well to force the molten anodic metal through said perforations to form in-situ, sacrificial metal anodes in electrical contact with said casing.

7. A method in accordance with claim 6 in which the anodic metal used is an alloy melting at a temperature less than about 660 F.

8. A method. in accordance with claim 6 in which excess amounts of the low-melting alloy are removed from the casing bore after resolidification.

9. A method in accordance with claim 6 in which said heating means is an electric heater. i

10. A method'in accordance with claim 6 in which the well casing and surrounding formation is heated to a temperature just above the melting point of said anodic metal prior to the melting of said metal and application of pressure to force the molten rri'etal through said perforations into the surroundingformation.

References Cited in the file of this patent UNITED STATES PATENTS 1,613,461 Johnson Jan. 4, 1927 1,646,735 Mills Oct. 25, 1927 1,804,078 Baden May 5, 1931 2,157,180 Little May 9, 1939 2,298,129 Irons Oct. 6, 1942 2,304,372 OBannon -2. Dec. 8, 1942 2,363,269 Schlurnberger Nov. 21, 1944 2,846,385 Buchan Aug. 5, 1958 

1. A METHOD OF PROVIDING CATHODIC PROTECTION TO THE EXTERIOR SURFACE OF METALLIC WELL CASINGS, WHICH COMPRISES FORMING PERFORATIONS IN THE CASING BELOW GROUND, PLACING A LOW-MELTING METAL ANODIC TO TEH CASING ADJACENT TO SAID PERFORATIONS, MELTING SAID METAL, AND APPLYING GAS PRESURE TO THE WELL TO FORCE THE MOLTEN ANODIC METAL THROUGH SAID PERFORATION TO FORM IN-SITU, SACRIFICIAL METAL ANODES IN ELECTRICAL CONTACT WITH SAID CASING. 